1. Field of the Invention
The invention pertains to the production of polydimethylsiloxanes containing T-aminoalkylsiloxy groups.
2. Background of the Invention
Organopolysiloxanes containing aminoalkyl “T” units which serve as branch points in the polymer have many uses, for example as textile additives to improve softness and hand. Such organopolysiloxanes can also be used as curing agents in numerous polymer systems, such as polyurethanes, epoxy resins, polishes and the like.
These aminoalkyl-functional organopolysiloxanes (aminoalkyl-functional “silicones”) have been produced by several methods in the past, but a preparation based on condensation of a silanol-stopped organopolysiloxane with an aminoalkyltrialkoxysilane has proven useful, as the raw materials are readily available at reasonable cost. During the condensation, silanol groups react with alkoxy groups in the presence of a catalyst to form a siloxane bond, with elimination of alcohol:≡Si—OH+MeO—Si(OMe)2R—NH2→≡Si—O—Si(OMe)2—R—NH2+MeOH
While beneficial for its overall simplicity and low raw ingredient costs, the method described above has some drawbacks as well. Due to the high alkoxy functionality of the aminoalkyltrialkoxysilane and also its initial reaction products, which contain unreacted alkoxy groups, the reaction mixture may exhibit an undesirable high viscosity, or may even gel, particularly if traces of water are present. Moreover, it is generally necessary to deactivate the catalyst once the desired molecular weight is obtained; otherwise, viscosity will continue to increase, and both initial product characteristics as well as storage stability may suffer. The aminoalkyl group-functional silane starting materials are widely used as adhesion promoters in numerous curable silicone compositions, and it has been noted, for example, that they appear to have the effect of a co-catalyst when used with conventional catalysts such as tin compounds. Whether due to this or for some other reason, it has been found that preparing T-aminoalkyl-functional silicones of reproducible viscosity is difficult. Minor process parameter excursions may result in a product with unusually high viscosity, to the point that the product may gel in the reactor itself, or may gel immediately following production.
It would be desirable to provide a method for the industrial preparation of T-aminoalkyl-functional silicones which avoid the aforementioned problems, which can produce T-aminoalkyl-functional silicones with a targeted viscosity in a reproducible manner.